From the publication of European Pat. No. 0,133,090, a transparent sheet of high optical quality able to be used alone or in association with other materials and particularly in the production of the previously described laminated glazings is known. This sheet comprises a layer made in a continuous process by reactive casting, on a horizontal plane from which it can be removed, of a reaction mixture of an isocyanate component and an active hydrogen component, particularly a polyol component. The isocyanate component preferably comprises a cycloaliphatic, an aliphatic diisocyanate, or a diisocyanate prepolymer, and it preferably has a viscosity less than about 5000 centipoises at +40.degree. C. The polyol component preferably comprises a difunctional long polyol with a molecular weight between 500 and 4000 and a short diol as a chain lengthening agent. By "reactive casting" is meant casting in the form of a layer or a film of a liquid mixture of components in the monomer or prepolymer state, followed by a polymerization of this layer by heat.
The known reactive casting obtains a layer of high optical quality in a single operation. However, one of the drawbacks of this production process is the necessity of using a long polymerization tunnel to obtain a complete polymerization and thus to avoid a marking on winding at the end of the line or, further, an additional treatment to prevent this marking.
Polyurethane sheets that can be used in laminated glazings can also be produced by extrusion. However, the extruded sheet does not directly have optical quality. Optical quality is obtained later during assembly with other elements of the glazing. When the extruded polyurethane sheet is used as an inner layer, the optical quality is obtained at the time of the assembly by pressing with the other elements of the glazing. When the polyurethane sheet is used as an outer layer in an asymmetric laminated sheet, optical quality can be obtained using the countermold assembly process. For this purpose, a glass sheet is used with a shape identical to the one constituting the rigid element of the glazing in the case of a flexible plastic-glass glazing of polyurethane. The glass sheet is placed against the free face of the flexible sheet, and the unit is pressed at the assembly temperature of the laminated glazings (i.e., at about 120-130.degree. C.) for about 1 hour.
One of the drawbacks of the countermold process, particularly in the production of a laminate glazing of glass-flexible sheet, is the necessity of using as a countermold a second glass sheet with the same shape as the glass sheet entering into the composition of the glazing and which for this purpose is curved simultaneously with the glazing, and which after use is scrapped.
Another drawback of the countermold process is that, in certain cases, the optical quality can be only temporary. That is, defects placed in "memory" in the glass can reappear after several days or several months.